1. Field of the Invention
The present invention is directed to systems, methods, apparatus and related software to achieve improved electrostatic tacking and/or pinning. More particularly, the invention relates to charging systems and methods for applying optimally effective charge to objects such as discontinuous trains of printed matter and/or continuous ribbons. Accordingly, the general objects of the invention are to provide novel systems, methods, apparatus and software of such character.
2. Description of the Related Art
Electrostatic charging is used in several manufacturing processes in commercial printing, including electrostatic ribbon tacking and stack tacking. Electrostatic ribbon tacking is the preferred method to meet increase speed and efficiency in folding and cutting processes. The technology uses an electrostatic charge to hold multiple ribbons together, making them behave like a single web and preventing the leading, side or trailing edges of the signature from “peeling away” from the signature package. This allows the electrostatically-bonded ribbons to be cut with the required precision, as the individual ribbon tensions are equalized. Electrostatic ribbon tacking enables the pressroom to deliver crisply folded signatures to the bindery without “dog-eared” edges at speeds of up to 3,000 ft/min.
Electrostatic charging of the type discussed herein may be achieved with at least one conventional charge applying device which may take the form of a charging bar configured to operate in conjunction with a grounded roller to charge objects and/or a continuous web (such as material for manufacturing gusseted bags) as they move through the charge applying device (i.e., between the charging bar and the grounded roller).
Electrostatic charging of the type discussed herein may also be achieved using a charge applying device that takes the form of two conventional charging bars (or other conventional ionizing electrodes known in the art) facing each other, one on each side of the multi-ribbon web or discontinuous product train. A dual-polarity high voltage charging power supply may apply a positive voltage to one bar and negative voltage to the other. Airborne ions of opposite polarity are produced by the opposing bars and stream between the charging elements of the charge applying device toward the web moving therebetween and causing all the ribbons to hold tightly together. FIG. 1 shows a charging system 100 with two possible locations for electrostatically pinning/tacking plural ribbons (or layers) together: after folding, downstream of the nips or before folding, near the roll at the top of the former. As is known in the art, the distance between the two elements of a charge applying means will typically range from about 0.5 inches and about 6 inches and the ionizing voltage will typically range from about 5 kV to about 60 kV (with 10 kV to 30 kV being the most common range).
Electrostatic stack tacking is used in compensating stackers where belts convey magazines up the stacker to be dropped into a compensator where they are stacked to varying heights that meet postal routing specifications. Magazine stacks must move quickly through the compensator to keep up with the upstream equipment. When the stacks are pushed onto the conveyor or rollers leading to a shrink wrap tunnel or other packaging equipment, the stack must stay straight and integral without shifting. However, magazines with UV-coated covers, either perfect bound or saddle stitched, have slippery surfaces that make them prone to shifting. In addition, high page count saddle-stitched magazines are challenging since the spine side is thicker than the open side and this can cause books to slide over toward the open side and “shingle over” as they exit the compensator.
Electrostatic force of attraction can preserve the neat stack or block achieved in the compensating stackers and this is generally known as incline stack-tacking. Incline tacking systems, such as system 100′ of FIG. 2, typically use one charge applying device that includes a pair of charging bars, one placed above the magazine's path and the other placed below. The ionizing electrodes in the bars are normally aligned with and face each other. A positive voltage is applied to one bar and a negative voltage to the other using either a pair of high voltage charging generators (high voltage power supply) or a single dual-polarity power supply.
When the bars are energized with no products in the incline feeder, the opposite polarity air ions produced by the opposed bars will flow between the bars completing the electrical circuit. When magazines move between the bars, they interrupt this flow and ions of opposite polarity deposit on the front and back covers, leaving these surfaces oppositely charged. The moving products carry these charges away, as a “convection” electrical current, again completing the electrical circuit.
Magazines and other bound products formed of a plurality of sheets of material may be compressed by the electrostatic force between the front and back cover pages with the air being squeezed out. While that contributes to forming a neat integral stack, a secondary effect is most important. When a charged magazine is dropped into the stacker, it lands with its back cover on top of the front cover of the previous magazine. Opposing charges on the front cover of one magazine and the back cover of an adjacent magazine attract each other, causing the magazines to adhere to each other, as shown in stack 101 of FIG. 3. This attraction keeps the magazines from shifting when stack 101 is in motion.
The above-described use of conventional electrostatic charging systems can dramatically increase throughput rates. For example, production speeds on a Goss SP 2200 without incline tacking are typically only 175 to 200 per minute. When a conventional electrostatic charging system is properly installed in the feeder, however, throughput can exceed 300 books per minute. Nonetheless, further improvements and/or refinements to such systems are still possible.